1. Field of Invention
This invention relates to a demodulator for a frequency modulation signal (referred as FM signal hereafter) and more particularly to an FM signal demodulator for video signals, such as luminance signals, of video signal reproducing apparatus.
2. Description of the Prior Art
Recently, in the fields of video signal reproducing apparatus such as television receivers (referred as TV receivers hereafter) and/or video tape recorders (referred as VTRs hereafter), many improvements have been made for raising the quality of pictures or images on the image display screen of TV receivers or VTRs. As is well known, both the sharpness and the signal to noise ratio (referred as S/N ratio hereafter) of the picture are especially important factors for raising picture quality.
The sharpness of a picture is influenced by the frequency response characteristics, i.e., the signal waveform response characteristics of picture reproducing circuits in TV receivers and/or VTRs. For example, when the signal waveform response characteristics of the picture reproducing circuits are insufficient at a leading end portion and a trailing end portion of respective pulse-shaped signals, such as luminance signals in video signals, the resulting pictures on the image display screen become inferior in sharpness. As is well known, the luminance signal is included in a composite video signal together with other signals, such as color signals. In a record mode of VTRs, the luminance signal is frequency-modulated with a prescribed carrier signal (a resultant signal will be referred to as FM luminance signal hereafter). The color signal or chrominance signal is frequency-converted to a frequency band lower than the FM luminance signal band. The low frequency-conversion chrominance signal is superimposed on the FM luminance signal (a resultant signal will be referred as a record mode composite signal hereafter). Then the record mode composite signal is recorded on a magnetic tape. In a playback or reproducing mode of the VTRs, the composite signal reproduced from the magnetic tape is separated to the FM luminance signal and the low frequency-conversion chrominance signal. The FM luminance signal is frequency-demodulated and restored to the original luminance signal (referred as base band luminance signal hereafter). The low frequency-conversion chrominance signal is also frequency-converted to the original frequency band. The restored base band luminance signal and the chrominance signal are again combined so that the original composite signal (referred as base band composite signal hereafter) is obtained. The signal waveform response characteristics of a picture reproducing circuit is determined by the frequency transmission range of the circuit. Therefore, in order to obtain good signal waveform response characteristics, it is necessary to make the frequency transmission range of the picture reproducing circuits wider. In particular, it is desired to expand the frequency transmission range to a frequency as high as possible.
Many attempts have been made for raising the signal waveform response characteristics of picture reproducing circuits. However, it has become difficult to further increase the signal waveform response characteristics, because the frequency transmission range of the picture reproducing circuits has expanded to a relatively wide range as a result of recent progress in circuit design. Particularly, the improvement of the picture quality in VTRs by raising the signal waveform response characteristics has become difficult. This is because the frequency transmission range of the picture reproducing circuits in VTRs is restricted to a narrower rang than the range of such circuits in TV receivers.
Further, in VTRs an FM signal, recorded on the magnetic tape, has a property in that the FM signal is reduced its amplitude at a high frequency region due to a principle of magnetic tape recording. Accordingly, a reproduced FM signal is worsened its carrier to noise ratio (refered as C/N ratio hereafter) at the high frequency region in an input to output response characteristic.
Accordingly, the improvement of the picture quality has been attempted from the standpoint of the S/N ratio of the picture on the image display screen. However, an increase of the S/N ratio of the picture occasionally is accompained by a reduction of the frequency transmission range, i.e., a deterioration of the signal waveform response characteristics of the picture reproducing circuits. For example, when an attempt is made to improve the S/N ratio, especially in VTRs, the signal waveform response characteristics deteriorate, so that a significant noise arises on the leading and/or trailing end portion of respective pulse-like signals, such as luminance signals in the FM signal band of the composite video signal. Therefore it is important to increase the S/N ratio while maintaining the signal waveform response characteristics at a prescribed level.
Conventionally, there are following several know methods for raising the S/N ratio of pictures in VTRs.
(1) Increasing the amount of emphasis in an emphasis circuit in a picture recording circuit, in advance of reproducing pictures.
(2) Increasing the amount of cancelling in a noise canceller circuit in a picture reproducing circuit.
(3) Increasing the signal component with a relatively high C/N ratio (carrier to noise ratio) in the FM signal, in other words a low frequency signal component which is lower than the carrier signal, for raising the S/N ratio of a baseband signal after FM demodulation.
The conventional methods, however, have drawbacks as described below. When attempting to increase the amount of emphasis, as in method (1), some frequency signal components fail to carry out white level clipping and dark level clipping, so that the signal waveform response characteristics deteriorate.
In method (2), the noise canceller circuit extracts the high frequency component from the luminance signal, reverses the phases of the extracted signals after limiting the amplitudes of the high frequency component by a limiter and then adds the extracted signals to the original luminance signal. Thus, a noise with a low level and a high frequency in the original luminance signal is cancelled by the noise in the extracted signals. When an attempt is made to increase the amount of cancelling, the S/N ratio of signals is improved at flat waveform portions of the waveform. However, the noise is not removed at waveform change portions in which the signal steeply changes over a large amplitude, and has a high frequency component. For instance, this may occur in the section where the signal changes from the dark level to the white level. Moreover, the dulation of such noise increases. Therefore, the signal waveform response characteristics deteriorate and the noise in the waveform change section becomes more pronounced.
In method (3), when the lower side band signal component, is increased, a reversal phenomenon of the picture signal from the dark level to the white level occurs more easily and, at the same time, the image quality declines at the portion where dark level changes to white level. More particularly, the portion of the waveform changing from the dark level to the white level is the portion where the carrier signal of the FM signal moves at the highest frequency. Consequently, the C/N ratio of the signal becomes worst at the waveform change portion. Therefore, in method (3), which does not use a signal component with a bad C/N ratio, as that above, although the S/N ratio in the flat waveform portion is improved, the waveform change portion where the signal changes from the dark level to the white level deteriorates. Incidentally, one of the causes of signal deterioration is the fact that when the signal at the portion changing from the dark level is frequency-modulated, a frequency of the FM luminance signal is located in the upper end of the FM signal transmission band. This is to avoid using components with a low C/N ratio. That is to say, in method (3), the amplitude and phase of the FM signals tend to be distorted in the transmission path. As a result, the waveform change portion from the dark level to the white level deteriorates, so that the noise in this waveform change portion becomes more pronounced.
As explained above, when an attempt is made to improve the S/N ratio of the luminance signal in prior art VTRs, the signal waveform response characteristics deteriorate and, moreover, the noise in the waveform change portion substantially increases. Therefore, the S/N ratio can only be set at a compromise between these two. As a result, the prior art VTRs have a problem in that the S/N ratio of the signal worsens in the portion of the waveform which changes from the dark level to the white level.
FIG. 1 shows method (3) used in the prior art VTRs. Graph (a) in FIG. 1 shows a waveform of a luminance signal just after a video signal has been restored to the base-band by demodulation. Graph (b) in FIG. 1 is an enlargement of section A of the waveform shown by graph (a) in FIG. 1.
As seen from graph (b) in FIG. 1, it is clear that there is a great deal of noise at the tip of the leading end portion (the portion where signal changes from the dark level to white level). When the luminance signal has passed through a de-emphasis circuit and a noise canceller circuit, the luminance signal with a waveform, as shown by graph (c) in FIG. 1 can be obtained. Graph (d) in FIG. 1 is an enlargement of section A of the waveform shown by graph (c) in FIG. 1. As is clear from graph (d) in FIG. 1, the noise on the tip of the leading end portion remains, and is not totally removed by the de-emphasis circuit or the noise canceller circuit. As a result, the contrast at the boundary of pictures presented on the image display screen is adversely effected by the noise, and this leads to deterioration of picture quality.